Pneumocystis pneumonia (PcP) remains a serious life-threatening respiratory fungal infection of immunocompromised patients, and one of the most common AIDS-defining illnesses in the US and the world. PcP-related mortality rates have changed little over the past two decades, likely due to our inability to adequately treat the infection without exacerbating immunopathogenesis. Adjunctive corticosteroids are used to suppress inflammatory injury during antibiotic treatment, but the benefit of these broadly acting agents is uncertain. The mechanisms by which Pc is recognized and cleared from the lung remain incompletely understood. Alveolar macrophages (AMs) are at the frontline of the host-pathogen interaction, and serve as important effectors of pulmonary host defense against Pneumocystis. Macrophages possess an array of PRR that have the potential to recognize Pc, but they are typically ineffective for host defense when CD4+ T cell help is not available. The reason for this is unknown, but it has been suggested that Pc may actively avoid or suppress macrophage mediated host defense to insure survival and transmission. Our laboratory has identified an inbred mouse strain which is unique in its ability to resist Pc infection in the absence of T cells. The resistance phenotype requires the presence of AMs, and can be overridden by reprogramming the resistant AMs to a susceptible M1 biased phenotype. The identification of resistant and susceptible macrophage phenotypes will provide an opportunity to explore the divergent host-pathogen interactions associated with either protection or infection. The overarching hypothesis of this proposal is that differential macrophage polarization, phagocytic processing of Pc, and antifungal effector production dictates the outcome of the Pc-AM interaction. To test this hypothesis we will utilize the resistant and susceptible mouse models described in our Preliminary Studies. The identification of new therapeutic strategies for the treatment of fungal diseases is an active area of drug-discovery research. Our long-term goal is to understand the mechanisms regulating macrophage mediated innate immunity in the lung to facilitate the rational design of therapeutic strategies to enhance host defense while limiting immunopathogenesis. To accomplish this goal we propose Specific Aims that will: 1) define functional differences in the phagocytic machinery of resistant and susceptible AMs that dictate the outcome of infection; 2) explore novel antifungal functions for chitinase-like proteins (Chi3l3) and TAM receptors (MerTK); and 3) map the Pc resistance locus and identify resistance-associated effector molecules that contribute to protective antifungal innate immunity. Our Preliminary Studies demonstrate that AMs can be programmed for innate protection against this opportunistic fungal pathogen, and suggest that modifying macrophage function may represent a viable strategy to enhance antifungal host defense.